Combined diesel-rankine cycle reciprocating engine

ABSTRACT

A Diesel engine in which a boiler is installed to extract heat from cylinder and air cooling systems as well as exhaust gases to generate steam at a rate proportional to the fuel flow and in which means are provided for introduction of the steam into the cylinder close to top dead center separately from the fuel, before during and after combustion, so that the steam injection does not disturb the ignition but actively modifies the combustion and afterburning process. The ratio of the steam to the fuel mass is preferably 1:1 to 3:1 and the steam temperature and pressure is as high as allowed by the properties of the materials used in the steam generating equipment. The steam system is adaptable to large Diesel engines and results in strongly reduced fuel consumption and very low pollutant emissions.

TECHNICAL FIELD

The invention relates to a piston reciprocating thermal engine wherecompression ignition, limited pressure (Diesel) and steam (Rankine)cycles are realized using the same cylinders at the same time.

BACKGROUND OF THE INVENTION

In limited pressure compression ignition (Diesel) engines liquid fuel isinjected into the compressed air in the cylinders. The bulk ofcombustion occurs on the stoichiometric surface of a diffusion flameseparating the high temperature oxygen rich and high temperature fuelrich regions. The presence of the high temperature oxygen rich zonesleads to production of large amounts of nitrogen oxides (NO_(x)) and thepresence of the high temperature fuel rich zones to soot (particulatematter) production. Thus Diesel engines are regarded by some as beinghighly polluting. The additional feature of the engines is that theexhaust gases are hot and the air and cylinder walls require coolingwhich causes substantial heat losses decreasing the thermal efficiencyof the engines. In Rankine cycle (steam) engines water is evaporated byheating and the generated steam used to produce mechanical energy.Diesel-Rankine cycle combination is often used to utilize the waste heatof the Diesel (top) engine but the Rankine cycle is typically realizedin a separate (bottom) engine. An example of such solution is describedfor instance in the U.S. Pat. No. 5,133,298. Clearly this is arelatively expensive and potentially difficult solution.

In a number of patents for instance: U.S. Pat. Nos. 4,377,934;4,406,127; 4,706,468; and U.S. Pat. No. 4,901,531 different versions ofthe bottom steam cycle are proposed in which at least one cylinder ofthe internal combustion engine is used as the steam power recoverydevice, which in fact is a modification of the classic combined cycleengines. In another patent: U.S. Pat. No. 4,433,548 the use of the samecylinders alternatively for combustion and steam power strokes isdescribed. In yet another patent: U.S. Pat. No. 4,409,932 injection ofthe steam during the power stroke i.e. after combustion in a gasolinecar engine is proposed. A negative feature of all these solutions isthat the emissions of the combined systems are the same as for theinternal combustion (top) engine. Many patents for example U.S. Pat.Nos. 3,761,019; 4,014,299; 4,027,630; 4,059,078; 4,391,229; 4,409,931;4,844,028 describe steam injection into the inlet manifold of engines tocontrol power, knock or NO_(x). However these solutions do notnecessarily provide increased efficiency. Some patents: U.S. Pat. Nos.3,948,235 and 4,913,098 propose to supercharge the engine using thesteam either by a steam driven compressor or compressor and ejector. Theefficiency increase of the solutions is relatively small so as it is forthe solutions described in U.S. Pat. Nos. 4,301,655 and 4,402,182 wherespecial cylinder heads where steam can be evaporated and injected intothe engines are described since in these solutions only the cylinderhead cooling heat is utilized in the power stroke. Premixing the fuelwith water, vaporizing the mixture and using the evaporated homogeneoussteam-fuel mixture as a modified fuel are described in the U.S. Pat. No.4,909,192. However, to realize the full potential of the system thesteam fuel mass ratio shall be in the range 1:3 and the steamtemperature above 500° C. The massive steam injection in the premixedmode can cause severe ignition problems whereas preheating of the fuelto the temperatures required even in the presence of water may lead tosubstantial fuel decomposition and coke formation in the fuel/steamsystem. Thus the solution does not seem to be feasible especially forheavy fuel operation.

SUMMARY OF THE INVENTION

The aforementioned goal is achieved using as much as possible of thewaste heat i.e. the air and cylinder cooling heat including cylinderhead cooling as well as the exhaust of the internal combustion (Diesel)engine to generate steam which is then injected back into the cylindersof the internal combustion (Diesel) engine close to the top dead centerso, that the steam actively changes the combustion process withoutinterfering with the ignition. In this way the maximum potential of theDiesel-Rankine cycle combination is achieved using the same cylindersand reciprocating mechanisms at the same time. This simplifies andreduces substantially the cost of the system. In addition the maximumtemperatures in the cylinders obtained during the fully developedcombustion are reduced which radically decreases NO_(x) emissions andthe mixing of combustion gases with remaining air enhanced duringafterburning which reduces soot emissions. To achieve the goals of theinvention over a range of engine loads the rate of water injection intothe steam generating apparatus should be proportional to the averagefuel flow, which maintains the proper engine power and steam massinjection balance. In another embodiment a small part of the steam,typically less than 10% of the fuel mass, is used for rapid atomizationof fuel without using expensive pressure fuel injection systems.

Thus the invention provides a simple yet effective solution to longstanding problems of the combined Diesel-Rankine cycle engines. Theforegoing and other objects and advantages of this invention will appearfrom the following detailed description, taken in connection with theaccompanying drawings of a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary view in a schematic form of a cylinder of aDiesel engine and engine embodying the features of this invention, thecomponents being arranged to depict the fuel and steam injection stageof the operating combined engine.

FIG. 2 is a fragmentary view of the steam (S) and fuel (F) jets when acommon injection system is used.

FIG. 3 is a fragmentary view of the steam (S) injection at a differentlevel to the fuel (F) sprays.

FIG. 4 is a fragmentary view of the steam (S) injection at the peripheryof the cylinder in the direction tangential to the wall.

FIG. 5 is a fragmentary view of the prechamber for initial combustion offuel (F) substituting for the direct fuel injection system 8 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For simplicity of description, the drawings illustrate one cylinder ofan engine which may include any desired number of cylinders. Asillustrated in FIG. 1 the engine contains a cylinder 1 in which a piston3 is shown connected in a conventional manner by a connection rod 2 to acrank shaft from which output power is to be taken. The piston is sealedby piston rings 4 and the cylinder is closed by a cylinder head 5 inwhich an inlet valve 6, outlet valve 7, fuel injection apparatus 8 andsteam inlet 9 are located. The steam inlet is controlled by a steaminjected valve 10 admitting steam at a selected time close to top deadcenter before, during and after combustion. The steam is generated in aboiler 12 shown symbolically in which steam tubes 11 and steam collector13 are located. The boiler uses heat from hot water used for cylinderwall and cylinder head cooling as well as inlet air cooling and exhaustgases. Water is pumped into the boiler through a water pipe 14 by a pump15 at a rate proportional to fuel pumping rate. The pressure of steam iscontrolled by the rate at which the water is supplied and time ofinjection. A small control volume 16 is provided before the steam inletvalve to which the steam enters through a flow restriction 17. Openingof the steam valve 10 causes a pressure drop in the control volume 16which is measured by a pressure sensor 18. Monitoring the pressure dropwill provide necessary information about operation of the valve. Inparticular sluggish operation or seizure of the steam injection valvesand steam leakages to cylinders can be detected in this way.

In the simplest way the present invention can be realized using a Dieselengine designed for dual-liquid/gas-fuel operation. In such a solution,liquid fuel (F) is injected separately through the standard injectionsystem whereas the steam (S) is injected through the injectors for gasfuel. This is shown in FIG. 2 where the separate fuel (F) and steam (S)jets are shown. In another solution shown in FIG. 3 the steam (S) isinjected at the bottom of the combustion chamber below the fuel (F)jets. In both cases the injector timing is arranged in such a way thatthe injection of steam does not interfere with the fuel injection untilignition is achieved. In another system shown in FIG. 4 the injection ofsteam (S) is realized at the cylinder periphery tangentially to thecylinder wall. The injection creates a rapidly swirling layer of steam.The inertia forces (radial acceleration) keep the steam at the peripheryand prevent the steam-air mixing until ignition occurs. Then due to thestrong perturbation of steam layer by the pressure pulse created byignition the steam layer will loose stability and will very quickly mixwith the gas. As the combustion process at this moment will be alreadyfully developed the mixing will not do any harm to it but at the sametime will be instrumental in the NO_(x) and soot emission control. Yetanother way to practically realize the invention is by dividing thecombustion chamber of the Diesel engine into two parts: one for ignitionand first stage combustion where no steam is admitted and the otherwhere the steam is injected and afterburning of the partially reactedfuel from the first stage occurs in steam-air mixture. This isillustrated in FIG. 5 where only the prechamber is shown replacing thefuel injection system 8 in FIG. 1. In the last two cases the steam maybe injected long time before the fuel as the ignition will not beperturbed by the steam.

Yet another system uses a small amount of the steam, typically less than10% to blast inject, and atomize the fuel. The system can be likened tothat used in early Diesel engines where compressed air was used toatomize the fuel. From the early Diesel engine developments it is knownthat the air blast injection systems work very well but compression ofthe air to very high pressures needed was a complicated and energyconsuming process here replaced by steam blasting using waste engineheat for steam production i.e. as the primary energy source.

Whilst the invention has been described with reference to certainembodiments, it is to be understood that the invention is not limited tosaid embodiments and may be widely varied within the scope of theappended claims.

The ratio of water vapor mass to fuel mass is preferably 1:1 to 3:1, thetemperature of steam below 580° C. and the pressure of steam below 180bar. It can be shown that depending on the amount and parameters ofsteam the use of steam as described above can provide an increase ofshaft thermal efficiency of the Diesel engines by a factor up to 1.32,reduce the NO_(x) emissions by a factor up to 4 and substantially reducesoot emissions. The engines are particularly beneficial for high power,stationary and ship applications where heavy and residual fuels areburned and where space requirements for the auxiliary steam generationsystems are not critical.

What is claimed is:
 1. A method of operating a compression ignition(Diesel)-Rankine cycle reciprocating engine having at least one cylinderin which fuel is burned, a cooling system, and an exhaust systemassociated with the cylinder for conducting exhaust gases from saidcylinder, said method comprising: removing heat from the cooling systemand from the exhaust gases and employing such heat to generate steam,and introducing said steam into said cylinder, and wherein said steamand said fuel are introduced separately into said cylinder and interactafter ignition of the fuel.
 2. A method according to claim 1, comprisinginjecting the steam into the cylinder at the periphery of the cylinderand tangentially to the cylinder wall.
 3. A method according to claim 1,comprising injecting the steam and the fuel through a common injectorbut in separate jets.
 4. A method according to claim 3, wherein saidseparate jets have different injection angles.
 5. A method according toclaim 1, comprising injecting the steam and the fuel at differentrespective levels.
 6. A method according to claim 1, comprisinginjecting the fuel into a separate chamber thereby preventing mixingprior to ignition.
 7. A method according to claim 1, wherein the ratioof water vapor mass to fuel mass injected is in the range of 1:1 to 3:1.8. A method according to claim 1, comprising superheating the steamprior to injection.
 9. A method according to claim 1, comprisinginjecting a portion of the steam in a manner such as to inject and blastatomize the fuel.
 10. A method according to claim 9, wherein saidportion of the steam has a mass less than 10% of the fuel mass injected.11. A method according to claim 1, comprising using an auxiliary heaterto modify the steam parameters.
 12. A method according to claim 11,comprising using the auxiliary heater to modify the steam parameters forsuperheated operation.
 13. A combined compression ignition(Diesel)-Rankine cycle engine having at least one cylinder, a fuelinjection means for introducing fuel into the cylinder, a coolingsystem, and an exhaust system associated with the cylinder forconducting exhaust gases from said cylinder, said engine comprising: afirst means associated with the cooling system and the exhaust systemfor recovering heat therefrom, a second means for employing therecovered heat to generate steam, and a third means for introducing saidsteam into the cylinder separately from the fuel such that the steaminteracts with the fuel after ignition of the fuel.
 14. An engineaccording to claim 13, having a steam injection valve for controllinginjection of steam into the cylinder, the engine further comprising aprechamber between the second means and the steam injection valve, theprechamber having a flow restriction orifice and a pressure monitoringsystem to control operation of the steam injection valve.
 15. An engineaccording to claim 13, comprising means for generating steam at a rateproportional to the rate of fuel flow.